New ChIrrNodeProxyToAsset, a irr::scene::ISceneNode inherited class for a custom node that will be inserted automatically in Irrlicht and contains triangle meshes reflecting the Chrono meshes.
This contains also a shared ptr to the corresponding chrono ChAsset, so it can update triangle coordinates at each Update()
New way of mapping chrono assets to Irrlicht nodes. An Update method is passed to the asset tree, too, for each update of the parent ChPhysicsItem.
Refactoring of Vector to ChVector, etc.
Fixed: the getType() in irrlicht_interface nodes was broken. const was missing, was not working as a overloading.
At each Update() of a ChPhysicsItem, ex. a ChBody, now also the tree of assets (ChAsset items, such as ChVisualization shapes) will get their Update() call.
The new GetPlate() method in ChConveyor coan be used to access the real collision shape , ex. for setting collision families.
Ex.
mconveyor->GetPlate()->GetCollisionModel()->SetFamily(2);   (not the mbody->GetCollisionModel()->SetFamily(2); )
etc.
New features in the FEM code. First tests with visualization of dynamical simulations with a 4 node tetahedron are OK.
The implementation is still incomplete, only 4 node tetahedrons can be displayed.
Merge branch 'develop' of https://github.com/projectchrono/chrono into develop

Conflicts:
	src/irrlicht_interface/ChBodySceneNode.h
	src/unit_FEM/ChElementTetra_10.h
	src/unit_FEM/ChElementTetra_4.h